Mobile data and voice communication continues to evidence significant growth. With increasing popularity of data and voice communication, it is more likely that the communication needs of a large number of users must to be met which are all located within a small area, a case referred to as dense crowd scenario in the art. Typical examples include sport arenas or large office buildings.
In order to increase data transmission performance and reliability, the so-called multiple input and multiple-output (MIMO) technology may be used in wireless radio telecommunication for transmitting information between a base station and terminals of users. MIMO systems may use multiple send and receive antennas for wireless communication at a base station. The MIMO technology forms the basis for coding techniques which use the temporal as well as the spatial dimension for transmitting information. The enhanced coding provided in MIMO systems allows a quality and data rate of the wireless communication to be increased.
In a massive MIMO system, the base station may include a large number of antennas, e.g. several tens or even in excess of one hundred antennas with associated transceiver circuitry. The extra antennas of the MIMO base station allow radio energy to be spatially focused which improves capacity and radiated energy efficiency.
In order to adapt the transmit signal at each individual antenna of the base station in accordance with the currently active receiving terminals a base station logic needs information about radio channel properties between the terminals and the antennas. A pilot signaling scheme can be used for this purpose which allows the base station to set configuration antenna parameters for transmitting signals so as to focus radio energy at terminals or for receiving radio signals, for example. In a conventional MIMO system, training sequences may be transmitted from all terminals within the cell and possibly also neighboring cells in a time slot which is dedicated to the respective terminal. The training sequences need to be orthogonal in order for the base station to identify the configuration parameters for the plurality of antennas for each of the one of the terminals in conventional systems. Orthogonality may be achieved by using time division multiple access (TDMA), code division multiple access (CDMA) or frequency division multiple access (FDMA) technologies or a combination thereof.
In case the MIMO system uses time division duplex (TDD) each terminal can transmit a pilot signal, which can be received by the antennas and analyzed by the base station logic. It will be appreciated that time slots are one example for orthogonal channels, with orthogonality being attained in the time domain. In order to not interfere with each other, a certain time period can be assigned in each system frame where each terminal may transmit its pilot signal. The time slots in which terminals may transmit their pilot signals in combination are also referred to as a pilot portion of a frame. The remaining time slots of the frame may be used for downlink (DL) and uplink (UL) data transmission, with the DL and UL transmissions being performed in a plurality of time slots which may follow the header of the frame, for example. The pilot signals may each include a training sequence, with the pilot signal received at the plurality of antennas of the base station being analyzed by the base station logic. The base station may use the results of the analysis to determine configuration parameters for transmitting signals via the antennas to the respective terminals.
Massive MIMO systems may be deployed in buildings such as office buildings, shopping malls and so on, sport arenas or other areas in which a large density of users can occur. In such environments a large number of terminals may be located in the cell served by the MIMO base station. The time required for the pilot signaling of the terminals in each frame may increase with the number of terminals. For a large number of terminals, the time required for all terminals to transmit their pilot signals may exceed the available pilot signaling time in each frame. While the pilot signaling time, i.e. the number of time slots allocated to pilot signaling, may be adjusted dynamically, the transmission of payload data would be negatively affected if the pilot signaling time was increased to much.